Preheat the oven to 425°F (220°C).
Roll the puff pastry dough out into a 12-14 inch (30-35 cm) circle on a lightly floured surface. It can be a rustic circle, no need for perfection. Place the dough onto a sheet of parchment paper or in a greased 12-inch cast iron skillet.
Halve and pit each peach, then cut each half into slices about ¼ inch (5 mm) thick. If peeling, do so before slicing and pitting (see notes for peeling and blanching methods). 500g g peaches, pitted
In a large bowl, toss the peach slices with honey, sugar, cornstarch, lemon juice, vanilla, and salt until everything is well coated. 30 g honey, 12 g granulated sugar, 15 g cornstarch. ½ tablespoon lemon juice, 1 teaspoon vanilla extract, ¼ teaspoon salt
Place the peach filling into the center of the dough, leaving a 2-3 inch (5-7 cm) border. You can arrange the slices neatly or just pile them in for a more rustic look.
Fold the edges of the dough up and over the filling, overlapping the folds as you work your way around the circle.
If you're not baking it in a skillet, transfer the galette on the parchment paper to a baking sheet.
In a small bowl, whisk the egg and water together. Brush the pastry edges with the egg wash. 1 large egg, 1 tablespoon water
Bake at 425°F (220°C) for 15 minutes, then turn the oven down to 350°F (180°C) and continue baking for around 40-45 minutes more, until the pastry edges are a deep golden brown and the peach filling is bubbly and thickened.
Let the galette cool for at least 20 minutes before serving so the juices can set. Drizzle with a little extra honey while still warm. Serve with vanilla ice cream or whipped cream. Extra honey for drizzling after baking